This invention relates generally to multiplexed amplifiers and, more particularly, to multiplexed amplifiers that operate at very low temperatures and are suitable for use on a space platform. Various X-ray and millimeter-wave cameras are under development for use in earth observation and space exploration. The most sensitive of these cameras are cryogenic. If the detector elements of a camera can be cooled below 10 Kelvin, the thermal mass of the individual pixels can be reduced to such a degree that individual photons can be detected by the resulting temperature rise of the corresponding detector elements.
Low operating temperatures dictate low available cooling power on the sensor or detector stage of these low-temperature cameras. In a detector stage having thousands of pixels, meeting these cooling constraints requires controlling the amount of heat leaking through wires connecting to the detector elements, and controlling the amount of heat dissipated in detector readout amplifiers. It has been recognized that controlling the heat leaked through the detector connecting wires and the heat dissipated in detector readout amplifiers can be effected by minimizing the number of connecting wires and readout amplifiers. Efforts have been made to reduce heat loads by multiplexing multiple detectors to shared amplifiers and wiring. For example, it has been proposed to use time division multiplexing (TDM) to sample up to 32 pixels of a detector stage sequentially through a common Superconducting Quantum Interference Device (SQUID) amplifier. Each pixel includes a SQUID on/off switch that performs the multiplexing operation. Another approach uses frequency domain multiplexing (FDM) to stimulate each of up to 32 pixels at a different frequency. The summed signal is amplified with a SQUID amplifier. Both these prior art techniques are significantly limited because only 32 pixels per amplifier may be multiplexed, and there is still a need to dissipate power in the SQUID circuitry.
Accordingly, what is needed is a multiplexer/amplifier that can handle many more than 32 pixels, can be conveniently located on the sensor platform, and will dissipate very low power. The present invention achieves these and other goals.